NEW YORK — Jake Mangum had been waiting a long time for the chance to play at Citi Field. Since June 4, 2019, to be exact, the day he was drafted out of Mississippi State in the fourth round by the Mets. “When I got drafted and spent four years with the Mets, this was where, like, your dream destination was,” Mangum said. “So to get here and play a game here, really cool. Great atmosphere. …